The collisions with the terrain by an aircraft that is still maneuverable termed CFIT (acronym standing for “Controlled Flight Into Terrain”) have been and remain one of the main causes of air disasters. To forestall them various types of airborne systems have been proposed.
The GPWS type systems (acronym standing for “Ground Proximity Warning System”), which were developed some thirty years ago, are based on the use of radioprobes making it possible to determine in an instantaneous manner a position or a dangerous tendency to approach the ground on the part of the carrier aircraft.
More recently the GPWS type systems have been replaced with more competitive systems of GCAS type (acronym standing for “Ground Collision Avoidance System”) also known under the generic term TAWS which rely on the detection of the possibilities of collision between the potential trajectories of the aircraft and the terrain overflown. These TAWS systems, which meet the international aeronautical standard TSO C151A, possess, in addition to the customary functions of the GPWS systems, a predictive function of alert of risk of collision with the relief and/or obstacles on the ground termed “FLTA” (acronym standing for “Forward Looking Terrain collision Awareness and alerting”) which delivers alerts and alarms to the crew so that an avoidance maneuver is engaged when a situation of risk of collision with the terrain arises.
The FLTA function relies on a location fix of the aircraft with respect to the region overflown provided by a flight equipment such as: inertial platform, satellite-based positioning receiver, baro-altimeter, radio-altimeter or a combination between several of these sensors, and on the monitoring of the penetration into one or more deployment protection volumes tied to the aircraft, of a model of the relief and/or of the obstacles on the ground which is extracted from a digital map accessible from the aircraft.
As it involves detecting a penetration of the terrain overflown, the protection volumes tied to the aircraft are mainly defined by their lower and frontal surfaces which form probers and whose longitudinal profiles correspond to those of a standard avoidance maneuver trajectory engaged in the more or less short term on the basis of an extrapolation of the trajectory followed by the aircraft.
The very widely advocated avoidance maneuver corresponds to a pure vertical avoidance maneuver termed “Pull-Up”, which consists of a full-throttle climb preceded by a flattening out of the wings if the airplane was banking and which is termed the “standard avoidance maneuver” or else “SVRM” (acronym standing for “Standard Vertical Recovery Maneuver”).
For further details on the ideas implemented in the TAWS systems, useful reference may be made to American patents U.S. Pat. Nos. 5,488,563, 5,414,631, 5,638,282, 5,677,842, 6,088,654, 6,317,663, 6,480,120 and to French patent applications FR 2.813.963, FR 2.842.594, FR 2.848661, FR 2.860.292, FR 2.864.270, FR 2.864.312, FR 2.867.851, FR 2.868.835.
The protection volumes tied to the aircraft are in general two or more in number, of tiered sizes, the foreward most being used to give an alert (“Caution”) signifying to the crew of the aircraft that the trajectory followed will have to be modified in the medium term to avoid the terrain, and the closest being used to give alarms (“Pull-up”, “Avoid Terrain”) signifying to the crew of the aircraft that it must actually engage, as a matter of great urgency, an avoidance maneuver.
During an approach for landing, the systematic response of a crew without outside visual reference, to an alert (“Caution”) of a TAWS system is the interruption of the approach maneuver for the engagement of a standard terrain avoidance maneuver with a view to bringing the aircraft to a safety altitude where a new approach procedure can be initiated in complete safety. This response to the detected risk of collision which involves a renewal of the approach procedure is particularly constraining while perhaps, a simple trajectory stabilization maneuver would have sufficed to deal with the risk. It is also constraining, but to a lesser extent, while gaining a cruising altitude after takeoff.
There therefore exists a requirement to better characterize an alert (“Caution”) of a TAWS system to allow a crew to better proportion its maneuver to the detected risk of collision with the terrain.